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    Re: Computer space required to get 1" accuracy
    From: Paul Hirose
    Date: 2010 Apr 08, 16:45 -0700

    Antoine Couette wrote:
    > - We start with standard data as available from any Modern and recent
    > Numerical Integration ( let's take the best current one from JPL : their
    > magnificent DE421 ) and we are authorised to "drop" some of their
    > published figures and published results in order to save some computer
    > space, as long as we fully guarantee 1" accuracy anyday anytime. We are
    > using the data as they are published (i.e. I would assume the X,Y,Z
    > Variables in the Ecliptic 2000.0 reference frame).
    DE423 is available for download at the JPL FTP site. I have not tried it
    or found any description of this ephemeris, however. Its time span is
    1800 - 2200.
    DE421 goes from 1900 to 2050. My software can use any ephemeris from
    DE102 to DE421, but I have never downloaded DE421 because there's only
    one file for the whole time span, and it's about 40 MB. That's a long
    download on a low speed connection. Also, the ephemeris time span is not
    adequate for investigating many historic observations.
    The DE405 and later ephemerides generate xyz coordinates with respect to
    the ICRF. To .1" accuracy, this is identical to the J2000.0 mean equator
    and equinox.
    I said the DE421 file is a 40 MB download. That's the ASCII file; the
    binary should be smaller. But the binary at the JPL site won't work with
    the JPL Fortran software on a Windows system. You must download the
    ASCII ephemeris and convert it to binary on your system. I think all the
    JPL ephemerides are available in ASCII now.
    I filter out messages with attachments or HTML.

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